1. Field of the Invention
This invention relates to a process for producing carbamic acid esters from an amino group-containing compound, carbon monoxide (CO) and a hydroxyl group-containing compound in the presence of an oxidizing agent.
2. Description of the Related Art
Carbamic acid esters are important as raw materials for pesticides or precursors of isocyanates. Isocyanates have been broadly used as raw materials for urethane products such as soft or hard foams, paints and varnishes, waterproofing agents, adhesives, elastic fibers, etc. Particularly in the case of diphenylmethane diisocyanate (MDI), its new use applications as heat-insulating materials, impact-resistant, light materials for automobiles, etc. has also been extensively developed and its need has been expanded to a large extent.
Carbamic acid esters have been prepared from an isocyanate prepared by the reaction of an amine with phosgene (reaction equation 1) and an alcohol (reaction equation 2) as follows. EQU RNH.sub.2 +COCl.sub.2 .fwdarw.RNCO+2HCl (1) EQU RNCO+R'OH.fwdarw.RNHCOOR' (2)
Phosgene has raised various problems that it is a deadly poison, it uses chlorine requiring a large amount of electric power, conversion process into phosgene is complicated and hydrochloric acid is by-produced in a large amount. Thus, in order to simplify the process and save the energy, a process for producing carbamic acid esters without using phosgene has been researched. For example, in the case of preparation of phenylcarbamic acid esters, mainly three production methods have been researched. The first is a process of preparing a carbamic acid ester from nitrobenzene, CO and an alcohol at one step (reaction equation 3). The second is a process of preparing it from aniline or diphenylurea, nitrobenzene or oxygene, CO and an alcohol (reaction equations 4, 5 and 6). The third is a process of preparing it by first preparing diphenylurea from nitrobenzene, aniline and CO, followed by subjecting the resulting diphenylurea to alcoholysis (reaction equations 7-1 and 7-2). ##STR2##
In the reaction equation 3, as the catalyst therefor are used a metal of platinum (Pt) group or its compound and a chloride of compounds of transition metals such as Fe, V, etc. (Japanese patent publication No. Sho 53-33582/ 1978) and/or a tertiary amine such as pyridine (Japanese patent application laid-open No. Sho 51-125216/1976).
In the reaction equations 4 and 6, too, a metal of Pt group or its compound and a compound of transition metals such as Fe are used (Japanese patent application laid-open No. Sho 55-120551/1980). In the reaction equation 5, Pt black and I.sup.-1 are used (S. Fukuoka et al., Chem. Commun., 1984, 399). Use of transition metal chlorides as promotor raises problems in the aspect of slurrying the reaction solution and separating the product from the catalyst. Further, some catalysts using pyridine requires to make the concentration of expensive Pt group catalyst higher so that a problem of catalyst recovery occurs. On the other hand, in the reaction equations 7-1 and 7-2, no promotor is used, but Ru.sub.3 (CO).sub.12 cluster catalyst is used (Japanese patent application laid-open No. Sho 62-59252/1987). In this case, slurrying of the reaction solution raises no problem, but there are problems that a long time is required for the reaction and the catalyst activity notably lowers due to a small amount of H.sub.2 gas so that H.sub.2 -containing CO gas cannot be used. In a process where Pd or a Pd compound supported by a carrier and a porous substance are used as a main catalyst and a non-metal halide is used as a promotor in the reaction equation 5 (Japanese patent application laid-open No. Sho 57-122055/1982), slurrying of the reaction solution and separation of the catalyst from the product and catalyst recovery have no problem, but the yield of the product is low.